This invention relates to the use of polyether ionophore antibiotics to control bacterial growth during sugar (sucrose) production. It can be used with a wide variety of feedstocks such as sugar beet juice, sugar cane juice, hydrolyzed grain (e.g., corn or wheat) or any other starch or sugar-containing material that can be used to produce simple sugars.
One of the key steps in sugar production is an extraction process where feedstock such as sugar beets or sugar cane is treated to extract sugar (as an aqueous solution referred to herein as "sweet juice`.about.) from the plant material. For instance, in the case of sugar beets, a diffusion process is commonly employed where the beets are soaked in warm water. This is typically-performed at about 70.degree. C. under acid conditions (pH around 6) for a period of 1 to 2 hours. During that time, heat-tolerant bacteria can proliferate, feeding on sugar and thus reducing the amount that can ultimately be recovered and marketed. This negatively impacts plant productivity and is a significant problem for the industry. Sugar cane is commonly subjected to an extraction process involving milling in which similar problems are encountered.
The microcrganisms causing the problem are mostly gram positive bacteria that belong to the lactobacillus genus. Streptococcus, bacillus., clostridium, leuconostoc and pediococcus may also be present. In the past, formaldehyde has been used in an attempt to control bacterial growth, but this raises serious safety concerns.
This invention concerns a method for the production of sugar wherein a polyether ionophore antibiotic such as monensin, narasin, salinomycin, lasalocid, maduramycin or semduramycin is used to control or supress bacterial growth during the process. These compounds have good activity against gram positive bacteria and do not easily degrade over time or under high temperatures. This makes them attractive to the sugar industry because:
1. they remain active for many days under typical sugar plant operating conditions; and PA1 2. they remain active at the high temperatures and acid pH used in the extraction step.
The bacterial population in the extraction bath is greatly reduced by the addition of a bacteriostatic or bactericidal concentration, for example 0.5 to 3.0 ppm, preferably 0.5 to 1.5 ppm, of a polyether ionophore such as monensin. This control greatly reduces the bacterial consumption of sugar leading to a significant improvement in plant productivity. Surprisingly, there are no detectable polyether residues in the final white sugar crystals. This result is particularly important because it makes the invention suitable for manufacture of food grade, white sugar crystals.
1. Field of the Invention
THE KEY STEPS IN SUGAR PRODUCTION
The 4 main steps performed in a typical sugar plant are described hereafter.
EXTRACTION
The purpose of this step is to extract the sugar from the feedstock. It yields a sweet juice with a pH of about 6 that is very susceptible to bacterial contamination. It also extracts water-soluble substances such as proteins which must be removed from the medium since they can hinder sugar crystallization.
PURIFICATION
Its purpose is to eliminate organic substances extracted with the sugar. It consists in adding a mixture of lime and water to the sweet juice and then sending through a flow of carbon dioxide to precipitate calcium as calcium carbonate. After filtration, one gets a clear juice, with little organic content other than sucrose.
CONCENTRATION
This clear juice, which is about 14% sugar, is heated and concentrated into a syrup with a sugar content comprised between 60% and 70% by weight.
CRYSTALLIZATION
This last step yields white sugar and a byproduct, molasses. It consists in concentrating further the syrup at 85.degree. C. under vacuum to bring it beyond the saturation point of sucrose (in a state called "supersaturation"). Then, one introduces a small amount of sugar crystals (about 0.5 g) to trigger crystallization which spreads rapidly through the liquid, turning it into a mass of white sugar crystals bathing in a syrup colored by impurities. The white sugar crystals are separated by centrifugation, rinsed and dried.
This crystallization step is repeated twice on the non-crystallized syrup coming out of the centrifuge. The second and third time, it yields brown sugar that is not marketed. Instead, it is reinjected at the beginning of the crystallization phase with the syrup coming from the evaporation step to yield more valuable white sugar. Only white sugar is marketed.
After the third iteration, the dark, noncrystallized juice has become molasses. It contains about 50% sugar and 30% foreign matter that prevents further crystallization.
2. Description of a Preferred Embodiment